Greetings Anthony,

Since you've decided to sell this car, you might as well tinker with it while you still have it. The coolant leak you have from the thermostat housing is most likely caused by not installing the rubber seal that goes on top of the thermostat before installing the top housing. If you did install the rubber seal, then the three screws holding the top on are most likely not snugged up. The only other leak you could be having in that area would be coming from a hose clamp not being tight.
The heating problem, just from what I would guess could be the result of a misadjusted temp wheel rehostat. To do this fix you need an extremely thin piece of metal, I used a piece of roof flashing aluminum, worked great. It needs to be about 5 inches long and 1 1/2 wide cut little teeth with a tin snips in one end of it. When this is done you have your special tool. Start up the car and get it to operating temps, preferable to do this in the morning so you'll feel the heater temp difference when you adjust it. Looking at your temp wheel set the temp to full heat. Next slide the thin metal tool, teeth forward between the wheel and the dash housing on the right side of the wheel. Push in till you feel the teeth grab the gear that is on the shaft, then slowly move your temp wheel to half the heat setting while keeping tension on your home made tool. Remove your tool and move the temp wheel to high heat again and see if heat doesn't once again start to flow from the vents. Mine got out of adjustment from Summer to winter so I assume it's possible yours has too. No money involved in this fix. The water valve assembly in most cases, by looking at mine seems rugid enough unless just water was used in the cooling system to hold up for years of use. I wouldn't suspect it as the first cause of a heating problem.

Give these fixes a try while you're waiting, you may be surprised at what you will accomplish.
There might even be folks on this site around your area that require your computer skills as a trade off for their mechanical skills.

Charles

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"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan